Silver halide photographic elements for use in medical radiography commonly include silver halide emulsion layers coated on one side (single emulsion layer coated) or on both sides (double emulsion layer coated) of the support in order to minimize the X-ray dosage received by a patient.
Since silver halide emulsions are relatively inefficient X-ray absorbers, it is common use to associate the radiographic element with intensifying screens which absorb X-rays and emit near UV or visible light to which the radiographic element is sensitive or has been sensitized with spectral sensitizing dyes absorbed on the silver halide crystals.
The quality of image obtained upon exposure and development of said radiographic elements results negatively affected by light scattering and crossover exposure.
Light scattering occurs both in single and double emulsion layer coated radiographic materials when light emitted by one screen is diffused (scattered) by silver halide grains causing a reduction in image sharpness.
Crossover exposure, which also causes a reduction in image sharpness, occurs in double emulsion layer coated radiographic materials when light emitted by one screen passes through the adjacent emulsion layer and the support and, the light having been spread by the support, imagewise exposes the emulsion layer on the opposite side of the support.
Various means have been suggested to reduce light scattering and cross-over exposure, such as, for example, the use of dyes or other materials to alter the composition of silver halide layers, the introduction of optically separating barriers, e.g. undercoats which can be positioned between the silver halide layers or between the silver halide layer and the support, and the like. Such means, however, result in a significant reduction in the sensitivity of the radiographic element to the light emitted by the screens.
Recently high aspect ratio tabular emulsions have been described for use in double layer coated radiographic elements, see e.g. U.S. Pat. No. 4,425,425. Spectral sensitizing dyes are adsorbed on the surface of the tabular silver halide grains in an amount sufficient to substantially optimally sensitize said tabular silver halide grains, preferably spectral sensitizing dyes of the J-band sensitizing type. As is well known in the art (e.g., F. M. Hamer: Cyanine Dyes and Related Compounds, p. 710 (1964), Interscience Publisher), the sensitization maximum of a sensitizing dye in a silver halide emulsion (M-band sensitization maximum due to molecular absorption) lies near the absorption maximum of the free dye in an aqueous solution. Certain sensitizing dyes have a sharp sensitization band at a wavelength slightly longer (about 20 to 50 .mu.m) than the wavelength corresponding to the M-band sensitization maximum. This is known as J-band sensitization and the dyes are known as J-band spectral sensitizing dyes. It is also known that the J-band sensitization can be strengthened by adding water soluble iodides or bromides to the silver halide emulsions, such as described for example in The Photographic Journal, 90 B, 142 (1950); Zeitschrift fur wissenschaftliche Photographie, 53, 209, (1959); Photographic Science and Engineering, 13, 13-17, (1969) and in U.S. Pat. Nos. 3,865,598 and 3,864,134.
The sensitizing dye, adsorbed on the surface of tabular silver halide grains, acts also as a filter dye since it captures some of the radiation causing the cross-over exposure. Therefore, high aspect ratio tabular silver halide grains provide radiographic elements exhibiting reduced cross-over of exposure and therefore less reduction of sharpness attributable to cross-over.
A disadvantage that has been encountered with the use of high aspect ratio tabular silver halide grains is the stain due to the yellow-brown color of the developed silver image and to the residual spectral sensitizing dye which remains in the photographic element at the end of the processing, as described for example in U.S. Pat. No. 4,520,098 and in Japanese patent application S.N. J60/128,432. Such residual stain gives the image an undesired tone and disturbs the proper interpretation of the X-ray image. Attempts to reduce said stain by reducing the amount of the sensitizing dye adsorbed on the surface of the tabular silver halide grains causes a reduction in sensitivity.
Accordingly, there is still the need to provide low light-scattering and cross-over silver halide emulsion radiographic elements having reduced stain from the presence of spectral sensitizing dyes.